Site description
The most famous of the five northern islets of Mauritius, containing the last populations of several taxa extinct elsewhere, because it lacks rodents (and is one of the largest of all rodent-free tropical, high islands). It is a semicircular islet (rather than as its name suggests) composed of beds of welded tuff, with very steep slopes. Much of the islet is devoid of soil, because of weathering from wind and water, exacerbated by browsing and grazing by exotic mammals (goats and rabbits Oryctolagus cuniculus). Bare rock areas have been sculpted into numerous overhangs, small caves and pedestals. The vegetation is the sole relict (albeit highly degraded) of the palm-savanna that formerly dominated the coastal areas of northern and western Mauritius. The vegetation includes many exotic species, but eradication (in the 1980s) of goats and rabbits has resulted in radical increases in regeneration and survival of native and exotic plants. The islet is a Nature Reserve and access is strictly forbidden to all but authorized researchers and management staff.

Key biodiversity
See Box for key species. The islet supports large numbers of seabirds, mostly burrow-nesting Puffinus pacificus. Also present is the entire Afrotropical and Indian Ocean breeding population of a Pterodroma petrel presently attributed to P. arminjoniana (but see ‘Comments on the inventory’). Other procellariid species recently confirmed in small numbers on Round Island (with evidence of breeding, attempted breeding or occupation of nest-sites typical of the species elsewhere) are Pterodroma baraui, P. nigripennis, Puffinus lherminieri, P. assimilis, P. carneipes and Bulweria bulwerii; Bulweria was nesting and P. nigripennis (two pairs, trapped) were in burrows, but the occasional visits by one or two P. baraui do not indicate that a permanent population is present. The site has the largest colony of both Phaethon rubricauda and P. lepturus in the Mascarenes.

Non-bird biodiversity: Plant community: Mauritian palm-savanna (only site). Plant species: many rare species including Latania loddigesii, Dictyosperma album var. conjugatum, Hyophorbe lagenicaulis (palms, latter two confined to Round Island), Lomatophyllum tormentorii (all E), Pandanus vandermeerschi (R). Reptiles: Phelsuma guentheri (E), Leiolopisma telfairi (V), Casarea dussumieri (E), Nactus serpensinsula (V), Gongylomorphus bojerii (endemic to Mauritian islets). The first three are restricted to Round Island, and a fourth such species, Bolyeria multocarinata, is now probably extinct (last recorded in 1975). Nactus serpensinsula is otherwise known only from Serpent Island (IBA MU013), where a different race occurs. There are several invertebrates endemic to Round Island, for example, the centipede Scolopendra abnormis (VU) and the scale-insect Asterolecianum dictyospermae (Homoptera), the latter known exclusively from one of the two surviving Dictyosperma palms, both of which are on Round Island. In view of such cases, Round Island has been described as having more threatened species per unit area than any other land area in the world.